Rotary drill bits are commonly used for drilling boreholes or wells in earth formations. Earth-boring rotary drill bits include two general configurations. One configuration is the roller cone bit, which typically includes three roller cones mounted on support legs that extend from a bit body. The roller cones are each configured to spin or rotate on a support leg. The outer surfaces of each roller cone generally include cutting teeth for cutting rock and other earth formations. These cutting teeth are frequently coated with a hardfacing material, such as a superabrasive material. Such materials often include tungsten carbide particles dispersed throughout a metal alloy matrix material. Alternately, receptacles are provided on the outer surface of each roller cone into which superabrasive inserts are secured to form the cutting elements. The roller cone drill bit may be placed in a borehole such that the roller cones are adjacent the earth formation to be drilled. As the drill bit is rotated, the roller cones roll across the surface of the formation and the cutting teeth crush the underlying earth formation.
A second configuration of a rotary drill bit is the fixed-cutter bit, often referred to as a “drag” bit. These bits generally include an array of cutting elements secured to a face region of the bit body. The cutting elements of a fixed-cutter type drill bit generally have either a disk shape or a substantially cylindrical shape. A hard, superabrasive material, such as mutually bonded particles of polycrystalline diamond, may be provided on a substantially circular end surface of each cutting element to provide a cutting surface. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutters. Typically, the cutting elements are fabricated separately from the bit body and secured within pockets formed in the outer surface of the bit body. A bonding material, such as an adhesive or a braze alloy, may be used to secure the cutting elements to the bit body. A fixed-cutter drill bit is placed in a borehole such that the cutting elements are in contact with the earth formation to be drilled. As the drill bit is rotated, the cutting elements scrape across and shear away the surface of the underlying formation.
The bit body of a rotary drill bit typically is secured to a hardened steel shank having an American Petroleum Institute (API) threaded pin for attaching the drill bit to a drill string. The drill string includes tubular pipe and equipment segments coupled end to end between the drill bit and other drilling equipment at the surface. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit within the borehole. Alternatively, the shank of the drill bit may be coupled directly to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit.
The bit body of a rotary drill bit may be formed from steel. Alternatively, the bit body may be formed from a particle-matrix composite material. Such materials include hard particles randomly dispersed throughout a matrix material (often referred to as a “binder” material.) Particle-matrix composite material bit bodies may be formed by embedding a metal blank in a carbide particulate material volume, such as particles of tungsten carbide, and then infiltrating the particulate carbide material with a matrix material, such as a copper alloy. Drill bits that have a bit body formed from such a particle-matrix composite material may exhibit increased erosion and wear resistance compared to similar bits made from steel, but generally have lower strength and toughness relative to drill bits having steel bit bodies.
While bit bodies that include particle-matrix composite materials offer significant advantages over all-steel bit bodies in terms of abrasion and erosion-resistance, the lower strength and toughness of such bit bodies limit their use in certain applications. In particular, particle-matrix composite materials are known to exhibit brittle facture when subjected to high strain-rate impact loading, such as loading at strain rates greater than 102 sec−1. In a drilling environment, such loading can occur during drilling without warning. It is known to result in fracture of blades or cutters and resultant failure of the drill bit. Such failures are costly, as they generally require cessation of drilling while the drill string, drill bit or both are removed from the borehole for repair or replacement of the drill bit.
Therefore, improvement of the particle-matrix composite to increase the toughness, strength or other properties to reduce the occurrence of brittle fracture during drilling would be desirable and would increase the applications where such bit bodies may be used.